Many prior art automotive alarm systems detect unauthorized entry into the vehicle. In order to detect such entry, these alarm systems require that each point of entry (hood, trunk, door) have associated therewith a switch (including in some cases a light activated by the switch) which would cause current to be drawn from the battery when the point of entry was accessed. Systems of this type have several disadvantages.
The most serious disadvantage of such systems is the large cost of installation. It is necessary to run wires from the enclosure containing the alarm system to a relatively large number of points throughout the vehicle. Installation costs may in many cases far exceed the cost of the electronic circuitry of the alarm system itself.
A further disadvantage of such voltage sensing systems is that with many car batteries in common use, the voltage may not remain constant For example, normal chemical activity within the battery which causes bubbling of the electrolyte may cause the battery voltage to change. This may generate false alarms. Further, the absolute value of the battery voltage cannot be used as an alarm criteria because the battery voltage may change from time to time or from vehicle to vehicle.
There are other automobile burglar alarm systems which sense current drawn from the battery. Generally this is done by using an inductive loop. However, such loops can respond only to changes in current, and not to the absolute value of the current being drawn. Thus, any significant change in current drawn by an accessory can trigger a false alarm. If the threshold for triggering is set high enough so that such false alarms are avoided, there is a significant probability of missing an event which should in fact trigger an alarm.
Another disadvantage of systems which use current loop detection is that arming may occur even though current is being drawn from the battery due to, for example, a door being open or a radio being on. Since only changes in current (and not steady state current conditions) are monitored, it is not possible for such systems to differentiate between a steady state current and zero current condition.
U.S. Pat. No. 4,553,127 to Issa discloses a control circuit disposed in a battery casing and connected in series with the battery so as to limit the amount of current supplied to the vehicle to only that necessary to operate accessories, unless the operation of the vehicle is authorized. A current sensor senses the level of current passing through the control circuit.
U.S. Pat. No. 3,706,966 to So et al. discloses a system where the onset of battery current is utilized to produce a voltage pulse which is employed to trigger a silicon controlled rectifier (SCR). Two of the leads to the SCR may be connected to the negative battery cable between the negative battery and ground. The apparatus of So et al. requires that when the motorist parks and locks the automobile he must close a switch to arm the burglar alarm system.